1. Field of the Invention
The present invention relates to human vision testing devices and methods, and more particularly to a vision test chart and/or series of vision test charts and associated test methodology that may be utilized to both assess and demonstrate the effect of blur reduction through optical correction, such as low cylinder astigmatism or high order aberrations correction, on visual quality.
2. Discussion of the Related Art
Visual acuity charts, such as Snellen charts, are the most widely utilized form of vision test in clinical practice. Standard visual acuity tests require that a patient identify letters of the alphabet of decreasing size from a predetermined position or distance. The smallest row of letters that may be read accurately indicates the patient's visual acuity.
The symbols on any acuity chart are known as optotypes. In a Snellen chart, the optotypes are designed as block letters with a particular geometry. Many modern visual acuity charts use a Sloan optotype, where the thickness of the lines equals the thickness of the white spaces between lines and the thickness of the gap in the letter “C”. The height and width of the optotype is five (5) times the thickness of the line.
During a standard eye exam, the visual acuity chart is often the only visual target for the patient to assess changes in the quality of vision correction with different levels of correction. Due to the fixed contrast, limited range of spatial frequencies, and lack of imagery, the visual acuity chart cannot demonstrate the real world vision quality that the patient will experience. Research into the subjective quality of images based on optical properties has shown that spatial frequencies which are lower than the usual threshold measured on a visual acuity chart are the most important aspect of the perceived quality of an image.
In addition to the simplicity of the content, the letters on a visual acuity chart are an incomplete representation of the visual stimuli an individual both experiences and sees in a typical day. The best representation the visual acuity chart gives is of the ability to read distant signs, but there are many other visual experiences in an individual's life. These experiences include reading and recognizing faces, seeing textures, trying to make out an object that may be partially obscured, and more.
In astigmatism, differential curvature in the optics of the eye causes the image to be blurred rather than focusing to a point. The amount and shape of the blur differs with each patient's prescription and/or particular condition. Higher levels of astigmatism are very noticeable and the benefit of its correction is clearly demonstrated by looking at a visual acuity chart. Low levels of astigmatism are not as noticeable to the individual, particularly when being assessed with a standard eye chart tests as described above, as these tests do not always provide a compelling demonstration of low level cylinder correction for treating astigmatism. When the patient is being fit for contact lenses, this often leads to practitioners choosing to fit astigmatic patients into spherical contact lenses. This is primarily due to a lack of perceived visual benefit of a toric contact lens that uses cylinder to correct astigmatic blur from both the practioner's and patient's perspective. The consequence of fitting an astigmatic patient into a spherical contact lens is reduced vision quality as compared to that which may be achieved by utilizing a lens for correcting the low cylinder astigmatism. Accordingly, there is a need for an eye test that demonstrates the benefit of correcting low cylinder astigmatism or other causes of blur.
In addition to blur caused by low levels of astigmatism, blur from other sources are less noticeable using a visual acuity chart than in everyday life. These sources include blur from multifocal corrective lenses, spherical aberration, and other high order aberrations. An eye test that demonstrates real world vision improvements may be utilized to help a patient choose the best multifocal design for them or decide on the benefit of correcting high order aberrations, all without leaving the office.